1. Technical Field of the Invention
This invention relates to a rolling bearing and, more specifically, it relates to a rolling bearing suitable to be used in the circumstance with sliding contact, under conditions with less lubricant and in the circumstance where the lubricant tends to be exhausted.
2. Description of the Related Art
Generally, a rolling bearing undergoes contact stress repetitively by a rolling motion between a raceway surface and a rolling surface. Particularly, a tapered roller bearing causes a slipping motion between a roller end face and a flange since the rolling surface of the roller is in contact with raceway surfaces of inner and outer rings, and the roller end face is in contact with the guide flange of the inner ring.
In view of the above, it has been required for the bearing material that it is hard, durable to load, and having long rolling life and favorable wear resistance against slipping and SUJ2 is generally used in a case of bearing steel. Further, in a case of a case hardening steel, necessary life and wear resistance are obtained by applying quenching or carbonitriding or carburizing treatment and then applying quenching to a steel material corresponding to SCR420 thereby increasing the hardness to 58–64 of HRC.
However, since the rolling bearing undergoes differential slip in a ball bearing, skew in the roller of a roller bearing, or slipping contact at a flange part, abnormal wear is caused by seizure or friction when the lubrication condition is severe. Then, wear particles are formed as foreign matters, and biting of obstacles lowers the rolling life or results in seizure to bring about a problem of early fracture.
As the countermeasure, it has been proposed, for example, a technique of controlling the surface roughness at the roller end face or the guide flange thereby avoiding direct metal contact between both of them as less as possible as described in Japanese Published Unexamined Patent Application No. 96660/1998, a technique of adding extreme pressure additives for improving the seizure resistance or wear resistance to a lubricant in a case where direct contact is caused on the surface as described in Japanese Published Examined Patent Application No. 79280/1993, or a technique of forming fine pits on the rolling surface or the raceway surface by electrolytic discharging fabrication and arranging them regularly to ensure the lubricant at a contact portion thereby improving the lubrication property as described in Japanese Patent Laid Open No. 240254/1993.
Further, in view of the material, a technique of constituting lubrication parts put under sliding contact with a material comprising graphite, cementite and ferrite as the main ingredient, defining the average particle size of graphite to 1–5 μm and the total content of graphite as a area ratio to 0.1–2.5% as disclosed in Japanese Published Unexamined Patent Application No. 80838/1999.
However, while it has been described for the technique disclosed in Japanese Published Examined Patent Application No. 79280/1993 that molybdenum dialkyl dithiocarbamate or the like is added as the extreme pressure additives to the lubricant, since metal contact tends to occur in a circumstance where lubrication is exhausted, there is a problem that the effect of the extreme pressure additives is not attained.
Further, for the technique described in Japanese Published Unexamined Patent Application No. 240254/1993, since the diameter for the fine pit depends on the diameter of a nozzle through which the electrolyte is jetted, the diameter of the fine pit is substantially restricted to about 100 μm and, in addition, there is a worry of fracture starting from the pit edge and the cost is remarkably increased inevitably since fabrication takes a much time.
Further, for the technique described in Japanese Published Unexamined Patent Application No. 80838/1999, while effectiveness to friction or wear is described, there is no consideration for the seizure and the rolling fatigue.
This invention has been developed for solving the foregoing problems and it intends to provide a rolling bearing capable of improving seizure resistance and reducing friction wear without deteriorating the rolling fatigue even in the circumstance causing differential slip or in the circumstance where lubrication tends to be exhausted.